Originally Posted by drswift

Because: in an urban environment, a mountain bike is much faster than a road bike.

I ride both. If I was on a roadbike, I don't stand a snowballs chance in hell of catching myself on a mountain bike - in a city race. Main advantages of a mountain bike in this scenario: ability to change direction quickly (try do a fast 180 on a road bike, ability to nip through traffic, ability to break fast and change direction, ability to jump over curbs, down stairs, nip around pedestrians etc.

A roadbiker has zero chance of catching an equally good rider on a mountain bike in a short city chase.